There are a wide variety of applications where it is of interest to know the real time when a particular event happens. For example, it is often times desirable to know the precise point in time when a jet aircraft is closest to a given spot on the runway.
U.S. Pat. No. 3,573,724 relates to a traffic flow detecting apparatus wherein the level of noise is compared to a reference level for determining traffic. U.S. Pat. No. 3,383,652 discloses an apparatus for determining the trajectory of aircraft involving the use of a track and a plurality of crushable detection elements. This system depends upon taxiing aircraft actually impacting on a detection element.
Other methods are proposed, such as the placement of a speaker as set forth in U.S. Pat. No. 3,855,571 which includes a loudspeaker mounted on each airplane for transmission of a coded high frequency acoustic signal while the plane is on the ground. That system is totally ineffective when one does not have control over the airplane's being detected. Finally, devices have been employed to listen to a jet engine to determine possible engine malfunctions and abnormal conditions. U.S. Pat. No. 3,315,522 discloses an engine sonic analyzer system for detecting mechanical faults and rotating parts of a high speed engine.
At the present time, however, no system exists which permits remote recognition of the approach of an aircraft for the purpose of determining the point in time when the aircraft is closest to a given point on the airfield.